You'd need to step up the 12VAC a bit before you can rectify and filter it back to 12VDC. Since you're only talking 100ma, that would help.
If this bike is utterly free of electrical bits, does it even have an alternator? If so, what is the raw AC output voltage of the alternator?
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner